1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a liquid crystal display (LCD) device, to obtain the process margin for stitching spots on a large-sized LCD panel, for improving yield.
2. Discussion of the Related Art
Demands for various display devices have increased with the development of an information society. Accordingly, many efforts have been made to develop various flat display devices such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescent displays (ELD), and vacuum fluorescent displays (VFD). Some species of flat display devices have already been applied to displays for various devices.
Among the various flat display devices, the liquid crystal display (LCD) devices have been most widely used due to the advantageous characteristics of thin profile, lightness in weight, and low power consumption, whereby the LCD devices provide a substitute for Cathode Ray Tubes (CRT). In addition to mobile type LCD devices such as a display for a notebook computer, LCD devices have been developed for computer monitors and televisions for receiving and displaying broadcasting signals.
Despite various technical developments in the LCD technology for finding applications in different fields, research in enhancing the picture quality of the LCD device has been, in some respects, lacking as compared to other features and advantages of the LCD device. In order to use LCD devices in various fields as a general display, the key to developing LCD devices depends on whether LCD devices can implement a high quality picture, such as high resolution and high luminance with a large-sized screen, while still maintaining lightness in weight, thin profile, and low power consumption.
A general LCD device includes an LCD panel for displaying a picture image, and a driving element for applying a driving signal to the LCD panel. The LCD panel includes first and second glass substrates being bonded to each other with a predetermined interval therebetween, and a liquid crystal layer injected between the first and second glass substrates. The first glass substrate (TFT array substrate) includes a plurality of gate and data lines, a plurality of pixel electrodes, and a plurality of thin film transistors. At this time, the plurality of gate lines are formed on the first glass substrate at fixed intervals, and the plurality of data lines are formed in perpendicular to the plurality of gate lines at fixed intervals. Then, the plurality of pixel electrodes, arranged in a matrix-type configuration, are respectively formed in pixel regions defined by the plurality of gate and data lines crossing each other. The plurality of thin film transistors are switched according to signals of the gate lines for transmitting signals of the data lines to the respective pixel electrodes. The second glass substrate (color filter substrate) includes a black matrix layer that excludes light from regions except the pixel regions of the first substrate, an R/G/B color filter layer displaying various colors, and a common electrode for obtaining the picture image. In the case of an In-Plane Switching (IPS) mode LCD device, the common electrode is formed on the first glass substrate.
Next, a predetermined space is maintained between the first and second glass substrates by spacers, and the first and second substrates are bonded to each other by a seal pattern having a liquid crystal injection inlet. At this time, the liquid crystal layer is formed according to a liquid crystal injection method, in which the liquid crystal injection inlet is dipped into a vessel containing the liquid crystal while maintaining a vacuum state in the predetermined space between the first and second glass substrates. That is, the liquid crystal is injected between the first and second substrates by an osmotic action. Then, the liquid crystal injection inlet is sealed with a sealant.
The LCD device is driven according to the optical anisotropy and polarizability of the liquid crystal. Liquid crystal molecules are aligned using directional characteristics because the liquid crystal molecules each has long and thin shapes. In this respect, an induced electric field is applied to the liquid crystal for controlling the alignment direction of the liquid crystal molecules. That is, if the alignment direction of the liquid crystal molecules is controlled by the induced electric field, the light is polarized and changed by the optical anisotropy of the liquid crystal, thereby displaying the picture image. In this state, the liquid crystal is classified into a positive (+) type liquid crystal having a positive dielectric anisotropy and a negative (−) type liquid crystal having a negative dielectric anisotropy according to electrical characteristics of the liquid crystal. In the positive (+) type liquid crystal, the longitudinal (major) axis of a positive (+) liquid crystal molecule is disposed in parallel to the electric field applied to the liquid crystal. In the negative (−) type liquid crystal, the longitudinal (major) axis of a negative (−) liquid crystal molecule is disposed perpendicular to the electric field applied to the liquid crystal.
FIG. 1 is an exploded perspective view illustrating a general Twisted Nematic (TN) mode LCD device. As shown in FIG. 1, the TN mode LCD device includes a lower substrate 1 and an upper substrate 2 bonded to each other with a predetermined interval therebetween, and a liquid crystal layer 3 injected between the lower and upper substrates 1 and 2.
More specifically, the lower substrate 1 includes a plurality of gate lines 4, a plurality of data lines 5, a plurality of pixel electrodes 6, and a plurality of thin film transistors T. The plurality of gate lines 4 are formed on the lower substrate 1 in one direction at fixed intervals, and the plurality of data lines 5 are formed perpendicular to the plurality of gate lines 4 at fixed intervals, thereby defining a plurality of pixel regions P. The plurality of pixel electrodes 6 are respectively formed in the pixel regions P defined by the plurality of gate and data lines 4 and 5 crossing each other, and the plurality of thin film transistors T are respectively formed at crossing portions of the plurality of gate and data lines 4 and 5. Next, the upper substrate 2 includes a black matrix layer 7 that excludes light from regions except the pixel regions P, R/G/B color filter layers 8 for displaying various colors, and a common electrode 9 for displaying a picture image.
At this time, the thin film transistor T includes a gate electrode, a gate insulating layer (not shown), an active layer, a source electrode, and a drain electrode. The gate electrode projects from the gate line 4, and the gate insulating layer (not shown) is formed on an entire surface of the lower substrate. Then, the active layer is formed on the gate insulating layer above the gate electrode. The source electrode projects from the data line 5, and the drain electrode is formed opposite to the source electrode. Also, the aforementioned pixel electrode 6 is formed of a transparent conductive metal having great transmittance, such as indium-tin-oxide (ITO).
In the aforementioned LCD device, liquid crystal molecules of the liquid crystal layer 3 on the pixel electrode 6 are aligned with a signal applied from the thin film transistor T, and light transmittance is controlled according to the alignment of the liquid crystal, thereby displaying the picture image. In this state, an LCD panel drives the liquid crystal molecules by an electric field provided perpendicular to the lower and upper substrates. This method obtains great transmittance and a high aperture ratio. Also, it is possible to prevent liquid crystal cells from being damaged by static electricity since the common electrode 9 of the upper substrate 2 serves as the ground. However, in the case of driving the liquid crystal molecules by the electric field perpendicular to the lower and upper substrates, it is difficult to obtain a wide viewing angle.
In order to overcome these problems, an In-Plane Switching (IPS) mode LCD device has been recently proposed. Hereinafter, the related art IPS mode LCD device will be described with reference to the accompanying drawings. FIG. 2 is a cross-sectional view schematically illustrating the related art IPS mode LCD device. In the related art IPS mode LCD device, as shown in FIG. 2, a common electrode 13 and a pixel electrode 12 are formed in the same plane of a lower substrate 11. Then, the lower substrate 11 is bonded to an upper substrate 15 at a predetermined interval therebetween, and liquid crystal 14 is formed between the lower and upper substrates 11 and 15. The liquid crystal 14 is driven by an electric field formed between the common electrode 13 and the pixel electrode 12 on the lower substrate 11.
FIG. 3A and FIG. 3B illustrate the alignment direction of the liquid crystal when a voltage is turned on/off in the related art IPS mode LCD device.
FIG. 3A illustrates the related art IPS mode LCD device when the voltage is turned off. That is, an electric field parallel to the lower and upper substrates is not applied to the common electrode 13 or the pixel electrode 12. Accordingly, there is no change in alignment of the liquid crystal 14. For example, liquid crystal molecules are basically twisted at 45° to a horizontal direction of the pixel electrode 12 and the common electrode 13.
FIG. 3B illustrates the related art IPS mode LCD device when the voltage is turned on. That is, the electric field parallel to the lower and upper substrates is applied to the common electrode 13 and the pixel electrode 12, thereby changing the alignment of the liquid crystal 14. In more detail, the alignment of liquid crystal 14 is twisted more than 45° as compared to the alignment of the liquid crystal when the voltage is turned off. In this state, the horizontal direction of the common and pixel electrodes 13 and 12 is identical to the twisted direction of liquid crystal.
As mentioned above, the related art IPS mode LCD device has the common electrode 13 and the pixel electrode 12 on the same plane. Thus, it has advantageous characteristics such as a wide viewing angle. For example, along a front direction of the IPS mode LCD device, a viewer can have a viewing angle of 70° in all directions (i.e., lower, upper, left, and right directions). Furthermore, the related art IPS mode LCD device has simplified fabrication process steps, and reduced color shift. However, the related art IPS mode LCD device has the problems of low light transmittance and low aperture ratio since the common electrode 13 and the pixel electrode 12 are formed on the same substrate. Also, it is required to improve the response time by the driving voltage, and to maintain the uniform cell gap due to the small misalign margin of cell gap. That is, the IPS mode LCD device has the aforementioned advantages and disadvantages, whereby a user can select the mode of the LCD device according to the desired purpose.
FIG. 4A and FIG. 4B are perspective views illustrating an operation of the IPS mode LCD device on the turning on/off state. Referring to FIG. 4A, when a voltage is not supplied to the pixel electrode 12 or the common electrode 13, the alignment direction 16 of the liquid crystal molecules is identical to the alignment direction of an initial alignment layer (not shown). However, as shown in FIG. 4B, when the voltage parallel to substrates is supplied to the pixel electrode 12 and the common electrode 13, the alignment direction 16 of the liquid crystal molecules corresponds to the electric field application direction 17.
Hereinafter, a related art LCD device will be described with reference to the accompanying drawings.
FIG. 5 is a plane view illustrating a related art IPS mode LCD device. As shown in FIG. 5, the related art IPS mode LCD device includes a transparent lower substrate (not shown). The transparent lower substrate includes a plurality of gate lines 21 formed in one direction at fixed intervals, and a plurality of data lines 23 disposed perpendicular to the gate lines 21 at fixed intervals, to define a plurality of pixel regions. Also, a common line 21a is formed in the pixel region on the same plane as the gate line 21 in parallel, and a plurality of common electrodes 21b connected to the common line 21a are formed at fixed intervals in the same direction as the data line 23. At this time, the common electrodes 21b are formed in a zigzag pattern.
A plurality of thin film transistors are formed at respective crossing portions of the plurality of gate and data lines 21 and 23. The adjacent upper and lower pixel regions use the gate line 21 in common. Also, the two thin film transistors of the adjacent pixel regions use gate and source electrodes in common. The adjacent two thin film transistors include a gate electrode defined in one portion of the gate line 21, a gate insulating layer (not shown) disposed on the entire surface of the lower substrate including the gate electrode, an active layer 22 formed below the data line 23 on one portion of the gate line 21, a source electrode 23a projecting from the data line 23 and having first and second grooves, and first and second drain electrodes 23b and 23c formed in the first and second grooves at a predetermined interval from the source electrode 23a. In this case, the adjacent thin film transistors of the upper and lower pixel regions use the gate electrode, the source electrode 23a and the active layer 22 in common.
After that, a passivation layer (not shown) is formed on the thin film transistor, and a plurality of zigzag-patterned pixel electrodes 24 are formed on the passivation layer between the common electrodes 21b. The pixel electrode 24 is in contact with the first/second drain electrodes 23b and 23c. At this time, the liquid crystal positioned between the common electrode 21b and the pixel electrode 24 is aligned at the same direction by the electric field parallel to the substrates, thereby forming one domain. According to the method of the electric field parallel to the substrates, it is possible to fabricate a multi-domain LCD device having a plurality of domains within one pixel region, thereby obtaining a wide viewing angle.
The upper substrate is formed opposite to the lower substrate. The upper substrate includes a black matrix layer 31 that excludes light from regions except the pixel regions of the lower substrate, and R/G/B color filter layers (not shown) displaying various colors. At this time, the black matrix layer 31 overlaps with one portion of the common electrode 21b adjacent to the common line 21a and the gate line 21, and one portion of the outermost common electrode 21b adjacent to the data line 23.
In case of forming the black matrix layer 31, the border between the upper and lower opening regions is determined by the black matrix layer 31. Also, the border between the left and right opening regions is determined by the common electrode 21b adjacent to the data line 23. The upper/lower/left/right opening regions are determined with the black matrix layer 31 and the common electrode 21b, so that it has the problem such as changes of the opening regions according to the bonding margin of the upper and lower substrates.
Also, as the large-sized upper and lower substrates are formed with demands for large-sized LCD devices, the exposure process is carried out divisionally when forming a TFT array of the lower substrate. At this time, if a misalignment is generated between the regions by the exposure process, stitching spots generate on a screen. Accordingly, when carrying out the exposure process on the lower substrate, it is required to take the stitching margin into consideration. When forming the black matrix layer 31 on the upper substrate, it requires one exposure process.
However, even though it takes the bonding margin and the stitching margin into consideration when fabricating the related art LCD device, the upper/lower/left/right opening regions are changed in that the upper and lower opening regions of the large-sized upper and lower substrates are determined by the black matrix layer 31, and the left and right opening regions are determined by the common electrode 21b. Accordingly, the opening regions (opening areas) are different in the respective pixel regions, whereby a problem is created, such as non-uniformity of luminance on the entire LCD panel.